Method for depinning the fermi level of a semiconductor at an electrical junction and devices incorporating such junctions

ABSTRACT

An electrical device in which an interface layer is disposed in between and in contact with a conductor and a semiconductor.

RELATED APPLICATIONS

The present application is a CONTINUATION of U.S. patent applicationSer. No. 15/728,002, filed Oct. 9, 2017, which is a CONTINUATION of U.S.patent application Ser. No. 15/251,210, filed Aug. 30, 2016, now U.S.Pat. No. 9,812,542, which is a CONTINUATION of U.S. patent applicationSer. No. 15/048,877, filed Feb. 19, 2016, now U.S. Pat. No. 9,905,691,which is a CONTINUATION of U.S. patent application Ser. No. 13/552,556,filed Jul. 18, 2012, now U.S. Pat. No. 9,425,277, which is aCONTINUATION of U.S. patent application Ser. No. 13/022,522, filed Feb.7, 2011, now U.S. Pat. No. 8,431,469, which is a DIVISIONAL of U.S.patent application Ser. No. 12/197,996, filed Aug. 25, 2008, now U.S.Pat. No. 7,884,003, which is a DIVISIONAL of U.S. patent applicationSer. No. 11/181,217, filed Jul. 13, 2005, now U.S. Pat. No. 7,462,860,which is a CONTINUATION of U.S. patent application Ser. No. 10/217,758,filed Aug. 12, 2002, now U.S. Pat. No. 7,084,423, which is related toU.S. patent application Ser. No. 10/342,576, filed Jan. 14, 2003, nowU.S. Pat. No. 6,833,556, all of which are hereby incorporated byreference.

FIELD OF THE INVENTION

The invention relates generally to semiconductor processing andsemiconductor devices. More particularly, the invention relates to aprocess for depinning the Fermi level of a semiconductor at ametal-interface layer-semiconductor junction and to devices that employsuch a junction.

BACKGROUND

One of the most basic electrical junctions used in modern devices is themetal-semiconductor junction. In these junctions, a metal (such asaluminum) is brought in contact with a semiconductor (such as silicon).This forms a device (a diode) which can be inherently rectifying; thatis, the junction will tend to conduct current in one direction morefavorably than in the other direction. In other cases, depending on thematerials used, the junction may be ohmic in nature (i.e., the contactmay have negligible resistance regardless of the direction of currentflow). Grondahl and Geiger first studied the rectifying form of thesejunctions in 1926, and by 1938 Schottky had developed a theoreticalexplanation for the rectification that was observed.

Schottky's theory explained the rectifying behavior of ametal-semiconductor contact as depending on a barrier at the surface ofcontact between the metal and the semiconductor. In this model, theheight of the barrier (as measured by the potential necessary for anelectron to pass from the metal to the semiconductor) was postulated asthe difference between the work function of the metal (the work functionis the energy required to free an electron at the Fermi level of themetal, the Fermi level being the highest occupied energy state of themetal at T=0) and the electron affinity of the semiconductor (theelectron affinity is the difference between the energy of a freeelectron and the conduction band edge of the semiconductor). Expressedmathematically:φ_(B)=φ_(M)−χ_(S)  [1]where Φ_(B) is the barrier height, Φ_(M) is the work function of themetal and χ_(S) is the electron affinity of the semiconductor.

Not surprisingly, many attempts were made to verify this theoryexperimentally. If the theory is correct, one should be able to observedirect variations in barrier heights for metals of different workfunctions when put in contact with a common semiconductor. What isobserved, however, is not direct scaling, but instead only a much weakervariation of barrier height with work function than implied by themodel.

Bardeen sought to explain this difference between theoretical predictionand experimental observation by introducing the concept that surfacestates of the semiconductor play a role in determining the barrierheight. Surface states are energy states (within the bandgap between thevalence and conduction bands) at the edge of the semiconductor crystalthat arise from incomplete covalent bonds, impurities, and other effectsof crystal termination. FIG. 1 shows a cross-section of an un-passivatedsilicon surface labeled 100. The particular silicon surface shown is anSi(100) 2×1 surface. As shown, the silicon atoms at the surface, such asatom 110, are not fully coordinated and contain un-satisfied danglingbonds, such as dangling bond 120. These dangling bonds may beresponsible for surface states that trap electrical charges.

Bardeen's model assumes that surface states are sufficient to pin theFermi level in the semiconductor at a point between the valence andconduction bands. If true, the barrier height at a metal-semiconductorjunction should be independent of the metal's work function. Thiscondition is rarely observed experimentally, however, and so Bardeen'smodel (like Schottky's) is best considered as a limiting case.

For many years, the cause underlying the Fermi level pinning of thesemiconductor at a metal-semiconductor junction remained unexplained.Indeed, to this day no one explanation satisfies all experimentalobservations regarding such junctions. Nevertheless, in 1984, Tersoffproposed a model that goes a long way towards explaining the physics ofsuch junctions. See J. Tersoff, “Schottky Barrier Heights and theContinuum of Gap States,” Phys. Rev. Lett. 52 (6), Feb. 6, 1984.

Tersoff's model (which is built on work by Heine and Flores & Tejedor,and see also Louie, Chelikowsky, and Cohen, “Tonicity and the theory ofSchottky barriers,” Phys. Rev. B 15, 2154 (1977)) proposes that theFermi level of a semiconductor at a metal-semiconductor interface ispinned near an effective “gap center”, which is related to the bulksemiconductor energy band structure. The pinning is due to so-calledmetal induced gap states (MIGS), which are energy states in the bandgapof the semiconductor that become populated due to the proximity of themetal. That is, the wave functions of the electrons in the metal do notterminate abruptly at the surface of the metal, but rather decay inproportion to the distance from that surface (i.e., extending inside thesemiconductor). To maintain the sum rule on the density of states in thesemiconductor, electrons near the surface occupy energy states in thegap derived from the valence band such that the density of states in thevalence band is reduced. To maintain charge neutrality, the highestoccupied state (which defines the Fermi level of the semiconductor) willthen lie at the crossover point from states derived from the valenceband to those derived from the conduction band. This crossover occurs atthe branch point of the band structure. Although calculations of barrierheights based on Tersoff's model do not satisfy all experimentallyobserved barrier heights for all metal-semiconductor junctions, there isgenerally good agreement for a number of such junctions.

One final source of surface effects on diode characteristics isinhomogeneity. That is, if factors affecting the barrier height (e.g.,density of surface states) vary across the plane of the junction, theresulting properties of the junction are found not to be a linearcombination of the properties of the different regions. In summary then,a classic metal-semiconductor junction is characterized by a Schottkybarrier, the properties of which (e.g., barrier height) depend onsurface states, MIGS and inhomogeneities.

The importance of the barrier height at a metal-semiconductor interfaceis that it determines the electrical properties of the junction. Thus,if one were able to control or adjust the barrier height of ametal-semiconductor junction, one could produce electrical devices ofdesired characteristics. Such barrier height tuning may become even moreimportant as device sizes shrink even further. Before one can tune thebarrier height, however, one must tiepin the Fermi level of thesemiconductor. As discussed in detail below, the present inventors haveachieved this goal in a device that still permits substantial currentflow between the metal and the semiconductor.

SUMMARY OF THE INVENTION

The present inventors have determined that for thin interface layersdisposed between a metal and a silicon-based semiconductor (e.g., Si,SiC and SiGe), so as to form a metal-interface layer-semiconductorjunction, there exist corresponding minimum specific contactresistances. The interface layer thickness corresponding to this minimumspecific contact resistance will vary depending upon the materials used,however, it is a thickness that allows for depinning the Fermi level ofthe semiconductor while still permitting current to flow between themetal and the semiconductor when the junction is biased (e.g., forwardor reverse biased). By depinning the level, the present inventors mean acondition wherein all, or substantially all, dangling bonds that mayotherwise be present at the semiconductor surface have been terminated,and the effect of MIGS has been overcome, or at least reduced, bydisplacing the semiconductor a sufficient distance from the metal.Minimum specific contact resistances of less than or equal toapproximately 10 Ω-μm² or even less than or equal to approximately 1Ω-μm² may be achieved for such junctions in accordance with the presentinvention.

Thus, in one embodiment, the present invention provides an electricaldevice in which an interface layer is disposed between and in contactwith a metal and a silicon-based semiconductor and is configured todepin the Fermi level of the semiconductor while still permittingcurrent flow between the metal and the semiconductor when the electricaldevice is biased. The specific contact resistance of the electricaldevice is less than approximately 10 Ω-μm². The interface layer mayinclude a passivating material (e.g., a nitride, oxide, oxynitride,arsenide, hydride and/or fluoride) and sometimes also includes aseparation layer. In some cases, the interface layer may be essentiallya monolayer (or several monolayers) of a semiconductor passivatingmaterial.

In another embodiment, the interface layer is made up of a passivationlayer fabricated by heating the semiconductor in the presence ofnitrogenous material, for example ammonia (NH3), nitrogen (N2) orunbound gaseous nitrogen (N) generated from a plasma process. In suchcases, the interface layer may be fabricated by heating thesemiconductor while in a vacuum chamber and exposing the semiconductorto the nitrogenous material.

A further embodiment of the present invention provides for depinning theFermi level of a semiconductor in an electrical junction through the useof an interface layer disposed between a surface of the semiconductorand a conductor. The interface layer preferably (i) is of a thicknesssufficient to reduce effects of MIGS in the semiconductor, and (ii)passivates the surface of the semiconductor. Despite the presence of theinterface layer, significant current may flow between the conductor andthe semiconductor because the thickness of the interface layer may bechosen to provide a minimum (or near minimum) specific contactresistance for the junction. As indicated above, the interface layer mayinclude a passivating, material such as a nitride, oxide, oxynitride,arsenide, hydride and/or fluoride.

Further embodiments of the present invention provide a junction betweena semiconductor and a conductor separated from the semiconductor by aninterface layer configured to allow a Fermi level of the conductor to(i) align with a conduction band of the semiconductor, (ii) align with avalence band of the semiconductor, or (iii) to be independent of theFermi level of the semiconductor. In some or all of these cases, currentmay flow between the conductor and the semiconductor when the junctionis biased because the interface layer has a thickness corresponding to aminimum or near minimum specific contact resistance for the junction.For example, specific contact resistances of less than or equal toapproximately 2500 Ω-μm², 1000 Ω-μm², 100 Ω-μm², 50 Ω-μm², 10 Ω-μm² oreven less than or equal to 1 Ω-μm² may be achieved.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings.

FIG. 1 shows a cross-section of an un-passivated silicon surfacecontaining surface silicon atoms with dangling bonds.

FIG. 2 illustrates various energy levels for metals and semiconductorsand is labeled to show the work function of a metal and the electronaffinity of a semiconductor.

FIG. 3 shows an energy band diagram for a conventional metal-n-typesemiconductor junction and also illustrates the concept of a depletionregion formed in the semiconductor when the materials are brought intocontact with one another.

FIG. 4 illustrates band bending at a conventional metal-n-typesemiconductor junction.

FIG. 5 shows a semiconductor device containing a semiconductor materialhaving a surface across which electrical current flows during operationof the semiconductor device, and containing an interface layer formed onthe surface according to one embodiment of the present invention.

FIG. 6 shows an electrical junction containing an interface layer thatis disposed between a semiconductor and a conductor in accordance withone embodiment of the present invention.

FIGS. 7a, 7b, 7c and 7d show relationships between Fermi energy,conduction-band energy, and valence-band energy for an unpassivated.Schottky diode, a passivated Schottky diode in which MIGS are notremoved, an unpassivated Schottky diode in which MIGS are removed and apassivated Schottky diode in which MIGS are removed according to oneembodiment of the present invention, respectively.

FIG. 8 shows a curve of interface layer resistance versus interfacelayer thickness for an electrical junction containing an interface layerdisposed between a semiconductor and a conductor in accordance with oneembodiment of the present invention.

DETAILED DESCRIPTION

Described herein are processes for depinning the Fermi level of asilicon-based semiconductor (e.g., Si, SiC or SiGe) at ametal-semiconductor junction as well as devices that use such ajunction. As more fully discussed below, an interface layer isintroduced between the semiconductor and the metal. The interface layerfunctions to passivate the semiconductor surface (that is, terminatedangling bonds that may otherwise be present at the semiconductorsurface so as to assure chemical stability of the surface) and todisplace the semiconductor from the metal so as to reduce the effect ofMIGS.

As discussed more fully below, the present inventors have determinedthat for thin interface layers disposed between a metal and asilicon-based semiconductor (e.g., Si, SiC and SiGe), so as to form ametal-interface layer-semiconductor junction, there exist correspondingminimum specific contact resistances. Indeed, minimum specific contactresistances of less than or equal to approximately 10 Ω-μm² or even lessthan or equal to approximately 1 Ω-μm² may be achieved for suchjunctions in accordance with the present invention. To achieve such lowcontact resistances, a metal that has a work function near theconduction band of the semiconductor for n-type semiconductors, or awork function that is near the valence band for p-type semiconductors,is selected.

The Schottky barrier in such junctions is already minimized, meaningthat it is much less than the Schottky barrier presented by a junctionin which the Fermi level is pinned, generally near the middle of thesemiconductor's bandgap. The current versus voltage (IV) characteristicof these junctions is non-linear, generally having a slope thatincreases as the voltage increases, such that the derivative of currentwith respect to voltage is increasing with voltage. This results in adecreasing differential resistance (dV/dI) and a decreasing resistance(V/I). Thus, a junction that has high resistance or high differentialresistance near the origin of the IV characteristic (zero volts) mayhave significantly lower resistance or lower differential resistance athigher voltages.

The present invention achieves low resistance and low differentialresistance near the origin of the current-voltage characteristic for ametal-interface layer-semiconductor junction. Generally, the voltagearound the origin should be less than about 100 mV, or more preferablyless than about 10 mV for purposes of measuring, determining, orutilizing such junctions of low resistance. At higher voltages, thejunction resistance will be even lower. It is thus a feature of thepresent invention to set an upper bound on the resistance of a contact,where the upper bound occurs at low voltages.

It is further noted that in junctions where the Schottky barrier isminimized as described above, such that the Fermi level at the junctioninterface at zero volts lies at or near the conduction band edge orvalence band edge (for n- and p-type semiconductors, respectively), theIV characteristic will be nearly symmetric, especially at low voltage.In this case, the term “forward bias” is not defined in the usual senseof a diode wherein forward bias corresponds to the direction of bias forwhich greater current flows. Thus, in determining or utilizing lowresistance junctions of the present invention, the voltage may be eitherpositive or negative.

It is also possible (in accordance with a further embodiment of thepresent invention) to make junctions where the Schottky barrier ishigher than it would be if the Fermi level at the junction interfacewere pinned, usually around mid-gap of the semiconductor. Such junctionsare formed in the present invention between a metal with a workfunctionnear or substantially equal to the conduction band edge of a p-typesemiconductor, or between a metal with a workfunction near orsubstantially equal to the valence band edge of an n-type semiconductor.These junctions are diodes, in that little current will flow if then-type (p-type) semiconductor is biased positive (negative) with respectto the metal, and large currents will flow if the voltage is reversed.The low-current flow state is referred to as reverse bias, and thehigh-current flow state is referred to as positive bias.

Low resistance in the case of a diode is only relevant in forward biasconditions. In junctions created in accordance with the presentinvention the resistance contribution of the interface layer is smallerthan the resistance due to the Schottky barrier. That is, in forwardbias conditions for junctions created in accordance with the presentinvention, the transport of charge is limited mainly by the thermalemission of carriers from the semiconductor over the barrier at theinterface, and not by the tunneling through the interface dielectric.Thus, low resistance in the case of a diode refers to a resistance lowerthan the resistance presented by the Schottky barrier.

In certain applications of diodes, the ability to withstand high reversebiases may be more desirable than high current flow in forward bias.These applications would be considered high voltage/low powerapplications. In such cases, a low resistance is not essential, andjunctions created in accordance with still another embodiment of thepresent invention provide high-voltage diodes capable of withstandingvoltages higher than could otherwise be achieved if the Fermi level ofthe semiconductor in the junction were pinned.

The present invention is discussed below in terms of presently preferredembodiments thereof, however, this discussion is not meant to limit thescope of the invention. By studying the present disclosure, others ofordinary skill in the art may recognize equivalent procedures, materialsor structures that can be substituted for those described herein toachieve the same effect. The reader is advised and reminded that the useof such equivalents is deemed to be within the scope of the presentinvention. For example, where the discussion refers to well-knownstructures and devices, block diagrams are used, in part to demonstratethe broad applicability of the present invention to a wide range of suchstructures and devices.

I. Introduction and Definitions

The present discussion makes use of terms that, although well known inthe art, may not be familiar to all readers. Therefore, before beginninga detailed discussion of the present invention, it is helpful to definecertain terms and concepts. To understand the properties ofmetal-semiconductor junctions and the impact of the present invention,one must refer to some important energy scales, which are showngraphically in FIG. 2. The so-called vacuum level (E0) represents theminimum energy that an electron needs to possess in order to completelyfree itself from a metal or semiconductor. For a metal, the Fermi level(E_(F)) represents the highest occupied energy level for the material.That is, nearly all energy states below the Fermi level are filled,while nearly all states above the Fermi level are empty. The workfunction of the metal (ΦM) is then defined as the energy required tofree an electron at the Fermi level and mathematically it is thedifference between the vacuum level and the Fermi level. The workfunction is an invariant bulk property of the metal.

As illustrated in the diagram, semiconductors also have a Fermi level(E_(F)) and a work function (Φ_(S)), however, the work function is notan invariant property of the semiconductor. Because the Fermi levelvaries depending on the doping level in the semiconductor (i.e., therelative amounts of impurities introduced into the semiconductor crystalwhich change the electron and hole carrier concentrations), a separateparameter, the electron affinity (χ_(S)), is defined. The electronaffinity is an invariant property of the semiconductor and is thedifference between the vacuum level and the conduction band edge of thesemiconductor. In a semiconductor, almost all energy states are filledin the valence band (E_(V)) while the conduction band (E_(C)) is almostempty.

Now consider a conventional junction between a metal and an n-typesemiconductor that has a work function smaller than the work function ofthe metal (i.e., Φ_(S)<Φ_(M)). An n-type semiconductor is one in whichelectrons are the majority charge carriers (in p-type semiconductors,holes are the majority charge carrier). As shown in FIG. 3, because theFermi level in the semiconductor is higher than the Fermi level in themetal, electrons transfer from the semiconductor 310 to the metal 320when the materials are brought into contact. Thus, a depletion region(i.e., a region in which there are no free charge carriers) 330 formsnear the junction interface 340.

The formation of the depletion region gives rise to an electric fieldand so-called “band bending”, as one approaches the junction interfacefrom the semiconductor side (see FIG. 4). The band bending creates anenergy barrier (discussed above) that blocks further transfer ofelectrons into or out of the semiconductor. Similar barriers are formedfor a junction between a metal and a p-type semiconductor when the workfunction of the metal is less than the work function of thesemiconductor. However, in the case of a metal-n-type semiconductorjunction in which the work function of the semiconductor is greater thanthat of the metal or a metal-p-type semiconductor junction in which thework function of the semiconductor is less than that of the metal, nosuch energy barriers are created and the contact is said to be ohmic innature.

As discussed above, although Schottky first postulated that the heightof the energy barrier (Φ_(b)) formed at a metal-semiconductor junctionwas simply the difference between the work function of the metal and theelectron affinity of the semiconductor, experiments have not verifiedthis relationship. Instead a more complex explanation that takes intoaccount the effects of surface defect states, inhomogeneities and MIGSappears to provide more accurate estimates of barrier heights byexplaining the pinning of the Fermi level in the semiconductor. Thepresent inventors have created a technique which is believed to depinthe Fermi level of a Si-based semiconductor at a junction with a metal(and thus allow for control or tuning of the barrier height) by bothpassivating the semiconductor surface (to eliminate or at least reducethe effects of surface states and possibly inhomogeneities) anddisplacing the metal from the semiconductor (to eliminate or at leastreduce the effects of MIGS). This depinning is achieved by introducingan interface layer between the semiconductor and the metal to create asemiconductor interface layer metal junction, which still permitssignificant current to flow between the metal and the semiconductor whenthe junction is forward biased.

This latter point is important. As discussed further below, for contactswhere the energy bands of the semiconductor and the conductor align(i.e., where the Fermi level of the conductor aligns with the conductionor valence band of the semiconductor depending on semiconductor typeand/or contact application), if the interface layer is too thin, thespecific contact resistance of the junction increases because of thepresence of MIGS, resulting in an increased barrier height; thus,current flow is hampered. Conversely, if the interface layer is toothick, the specific contact resistance is again increased and one getslow current across the junction because of tunneling limitations. Thepresent invention achieves an interface layer that is thick enough toreduce or eliminate the effect of MIGS, while still thin enough topermit significant current flow.

II. Passivation of Semiconductor Surfaces

A common processing operation performed during semiconductor devicefabrication is silicon surface passivation. Surface passivation (whetherby an oxide or another material) chemically neutralizes and physicallyprotects the underlying silicon. For example, exposing a silicon surfaceto oxygen (under the appropriate conditions to grow a protective film ofsilicon dioxide) will allow the oxygen to react with the dangling bondsof the silicon surface to form covalent bonds that satisfy the surfacesilicon atoms' valency and render the surface fully coordinated. Thesecovalent bonds provide chemical stability to the silicon surface. Thecovalent bonds also tie up unbound charges that exist on the siliconsurface as a result of the discontinuation of the semiconductor crystalat the surface.

However, passivation with silicon dioxide has several significantdisadvantages. For example, silicon dioxide is a dielectric insulatorthat poses a significant barrier to the flow of current. Accordingly, alayer of silicon dioxide deposited or grown on a silicon surface maysignificantly reduce the ability for electrical current to flow throughthat surface. As a result, the use of silicon dioxide has been limitedin practicality to surfaces external to the active region ofsemiconductor devices through which current passes during deviceoperation (e.g., as a gate oxide layer). This disadvantage is compoundedby the fact that the silicon dioxide grows very rapidly and readily onthe silicon surface so that it is difficult to limit the growth to athin layer. Silicon dioxide is also a poor diffusion barrier tosemiconductor dopants such as boron.

Instead of making use of silicon dioxide then, in one embodiment thepresent inventors utilize a nitrided semiconductor surface to providechemical passivation. That is, a nitride layer is introduced topassivate the semiconductor surface by eliminating or at least reducingthe effects of surface states and possibly inhomogeneities. The nitridelayer also displaces the metal from the semiconductor and eliminates orat least reduces the effects of MIGS. The result of introducing thenitride layer as an interface between the semiconductor and the metal isa depinning of the Fermi level of the semiconductor. When the Fermilevel of the semiconductor is depinned, the Fermi level of the metal atthe interface will be that of the bulk metal, and will not be dependentupon the interface. In addition to the above, the present inventorspropose techniques for providing non-insulating, passivatedsemiconductor surfaces using materials other than nitrogen; for example,oxides, hydrides, arsenides and/or fluorides.

These developments have wide applicability in connection with thefabrication of Schottky diodes, Schottky-barrier transistors and otherelectrical components. For example, in Schottky diodes, the ability tocontrol the energy barrier height at the diode junction is important ifthe device is to be tailored to specific applications. Use of thepresent techniques allows for tuning of the barrier height. Further, forother three-terminal devices with Schottky-barrier-isolated channels,control of device characteristics is made possible through the presentinvention by allowing n- and p-type devices to be fabricated withoutdopants, relying instead on the use of metals with different workfunctions.

FIG. 5 shows a semiconductor device 510 that contains a semiconductor530 and an interface layer 520 formed on a surface 540 of thesemiconductor in accordance with the present invention. The termssemiconductor device, microelectronic device, monolithic device, chip,and integrated circuit are often used interchangeably in this field. Anyor all such devices may each contain an interface layer formed on asemiconductor surface in accordance with the present invention.

The semiconductor 530 contains a semiconductor material. The termsemiconductor material refers to a material having a bandgap that isgreater than about 0.1 eV and less than about 4 eV. The term bandgaprefers to an energy gap of forbidden energy levels separating theconduction band, which is an upper energy band that is mostly devoid ofelectrons and wherein electrons can conduct, and the valence band, whichis an energy band that is mostly filled with electrons and whereinelectrons cannot conduct. The semiconductor material may have a widerange of doping levels including no doping at all.

The semiconductor 530 has a surface 540 that is passivated by theinterface layer 520. In this context (and as used elsewhere herein) theterm passivation means the elimination or at least the reduction of theeffects of surface states due to defects or dangling bonds of thesemiconductor surface 540. Note that passivation does not, as apractical matter, require that all surface states be eliminated. Rather,it is the effect of surface states on the device properties that islimited or eliminated in passivation. Note further that the presence ofMIGS may be regarded as a surface state, however, as used herein theterm passivation is not meant to infer the elimination of MIGS (thoughin some cases, a passivation layer may have sufficient thickness toprovide a separation layer between the semiconductor and the metalsufficient to reduce or eliminate MIGS). The semiconductor 530 isoperable to be electrically coupled with a first voltage associated withthe semiconductor device 510 and to conduct electrical current 550across the passivated surface 540.

The interface layer 520 is formed on the semiconductor 530 and maycontain a passivation material that bonds to the semiconductor materialby way of a covalent (or other) bond formed between the passivationmaterial and the semiconductor material. For example, an atom ofpassivation material may covalently bond with a dangling bond of asurface silicon atom to fully coordinate the silicon atom and therebyhelp passivate the silicon atom. In some cases, the passivation materialmay be the sole component of the interface layer 520, while in othercases the interface layer 520 may be a compound layer that includes botha passivation layer and a separation layer. That is, the interface layerserves to (i) chemically passivate the semiconductor surface 540, and(ii) displace the semiconductor from the metal sufficiently to eliminateor at least reduce the effect of MIGS. As explained below, this maynecessitate including a separation layer in addition to a passivationlayer within the interface layer, depending on the passivation materialselected. Of course, the combination of the passivation layer and theseparation layer must be sufficiently thin to permit the low specificcontact resistances described herein.

Different passivation materials are contemplated. According to oneembodiment, the interface layer 520 is formed using a material that ispreferably selected from the group consisting of hydrogen (H), oxygen(O), nitrogen (N), arsenic (As), and fluorine (F) (that is, theinterface layer 520 may include a nitride, an oxide, a hydride, anarsenide and/or a fluoride). Other materials having chemicalcharacteristics or valences similar to these materials may also be used.Note that distinct separation layers (i.e., in addition to thepassivation layer(s)) may be needed where H, As, or F passivation layersare used, as these tend to form monolayer coverage, rather than a layerof a compound with Si of process-dependent thickness. In contrast,passivation layers made using N and/or O may not require distinctseparation layers, as these elements may form a layer of a compound ofSi with a thickness that can be varied depending on processing.

Different amounts of passivation material are contemplated to be usefulfor different embodiments of the present invention. Often, the interfacelayer 520 includes or is made up of a passivation layer with a thicknessof between approximately 0.1 nm and about 5 nm. For example, dependingupon the particular implementation, the thickness may be less than about1 nm, less than about 0.5 nm, less than about 0.2 nm, may be thethickness corresponding to a single layer or monolayer of passivationmaterial that is bonded to the semiconductor surface, or may even be thenumber of atoms of passivation material required to passivatesubstantially all the dangling bonds associated with the semiconductorsurface 540.

In some cases, passivation of the semiconductor surface 540 will includeremoving (or terminating) dangling bonds located proximate to thesurface of the semiconductor material, including those at the surface aswell as those within a few molecular dimensions from the surface. Thisprocess may stabilize the surface of the semiconductor material and mayimprove the controllability of subsequent fabrication operations.Passivation may also reduce the density of surface states that may existat the semiconductor surface as a result of the discontinuation of thesemiconductor crystal at the surface. This may improve consistency andperformance of the semiconductor device, inasmuch as such states areknown to interfere with proper device operation. For example, they mayprovide surface charge states that result in a pinning of the Fermilevel.

III. Forming Interface Layers

Exemplary methods for forming interface layers to provide (i)passivation of semiconductor surfaces, and (ii) displacement of thesemiconductor from the metal to eliminate or at least reduce of theeffects of MIGS within the semiconductor when in the presence of themetal (collectively referred to herein as depinning the Fermi level ofthe semiconductor) with hydrogen, fluorine or nitrogen are presentedbelow to further illustrate the concepts of the present invention. Otherpassivation materials may include arsenic, oxygen or an oxynitride, andin some cases such passivation layers are combined with separationlayers (e.g., made of an oxide) to complete the interface layer.

A. Hydrogen and Fluorine

An interface layer may contain hydrogen, fluorine, or both hydrogen andfluorine (e.g., in the form of a hydride and/or a fluoride). One methodfor forming an interface layer on a semiconductor surface with hydrogenand fluorine includes cleaning the semiconductor substrate with acleaning solution, immersing the cleaned substrate in a hydrogenfluoride solution (or other liquid containing hydrogen and fluorineions) having an effective concentration typically between about 1%-50%by weight, waiting an effective period of time, typically between aboutseveral seconds and about 5 minutes, removing the substrate from thehydrogen fluoride solution, optionally rinsing the substrate indeionized water, and blow-drying the substrate with nitrogen. Such amethod may form an interface layer containing hydrogen and fluorine thatare bonded (e.g., covalently) to the semiconductor surface.

It should be noted that long rinses in deionized water, generally longerthan about 30 seconds, might remove the hydrogen passivation. Thus,deionized water rinses might advantageously be kept to less than about30 seconds to maintain the hydrogen passivation of the surface. Also,the higher the concentration of the hydrogen fluoride during theimmersion, the greater the concentration of fluorine passivation.Finally, methods are also contemplated where the ratio of hydrogen tofluorine passivation is altered by removing either the hydrogen or thefluorine.

An interface layer formed in this fashion may be best suited forapplications where a subsequent metal layer is deposited over theinterface layer in a generally non-invasive fashion, for example using athermally evaporated source. Experiments by the present inventors todate suggest that using other approaches (e.g., plasma deposition) maycause damage to the thin (e.g., monolayer thick) interface layercontemplated as part of the present invention.

B. Nitrogen

In a further embodiment, an interface layer may contain nitrogen (e.g.,in the form of silicon nitride). One method for forming an interfaceover a semiconductor surface with nitrogen includes heating a substratecontaining the semiconductor surface in the presence of a nitrogenousmaterial (that is, a gas or other material containing nitrogen). Forexample, a substrate containing an exposed silicon surface may beannealed at a temperature between about 300° C. and about 750° C., whichis lower than temperatures conventionally used for Rapid ThermalNitridation (RTN), under a gaseous ambient having, for example, ammonia(NH₃) at some effective partial pressure. By exposed, we mean a cleansurface, free of everything except silicon. Such a method may form aninterface layer containing nitrogen, often in the form of a nitride,bonded to the semiconductor surface. Note that the present inventorshave observed indications suggesting that in these low temperatureconditions interface layer growth is self-limiting, depending only ontemperature.

According to another embodiment, an interface layer that includesnitrogen may be formed on an exposed surface of a semiconductor materialby a method that includes heating a semiconductor material to asubstantially high temperature under vacuum and exposing thesemiconductor material to a substantially small amount of a nitrogenousmaterial, such as ammonia. The method may include placing asemiconductor having an exposed semiconductor surface in a heatingchamber, pulling a vacuum of less than about one millionth of a Torr, ormore favorably an ultra high vacuum of less than 10−9 Torr, and thenheating the semiconductor in the heating chamber to a substantially hightemperature. The higher the vacuum, the longer the substrate may beheated without growth of an oxide from residual oxygen or water in thechamber. Thus, the process may include heating the semiconductor to atemperature that is between about 900° C. and about 1000° C., or higher,in an inert ambient. As desired, the semiconductor may be exposed tohydrogen gas, or an equivalent, to reduce any native oxide on thesemiconductor. These high temperatures may provide for greaterpassivation of the semiconductor surface as compared with results thatmay be achieved at lower temperatures.

Then, the heated semiconductor may be exposed to a substantially smallamount of a nitrogenous material, such as ammonia. This may includeexposing the semiconductor surface to ammonia for a substantially shortperiod of time. For example, the surface may be subjected to a burst orpulse of ammonia lasting for a time period between about 0.5 seconds andabout 5 seconds. Alternatively, the surface may be exposed to acontrolled, small amount of ammonia over an arbitrarily longer period oftime. In this way, the substantially small amount of ammonia will reactwith the surface to form a nitrogenous interface layer, such as anitride layer, thereon and then further growth of the interface layerwill cease. Then the semiconductor may be cooled from the substantiallyhigh temperature to ambient temperature and removed from the heatingchamber. Further annealing of the substrate and the grown nitride layermay also be performed in the vacuum chamber before removal, at asubstantially elevated temperature between about 700° C. and 1000° C.,or higher.

Advantageously, it has been unexpectedly observed that a process such asthat described above and incorporating substantially high temperatureexposure for substantially short periods may be used to controllablyform thin yet effective interface layers. That is, the present inventorshave observed that in the creation of thin interface layers that includenitrogenous materials, temperature appears to be a dominant factor incontrolling thickness. For example, by such methods effective interfacelayers may be formed having a thickness that is less than about 1 nm,less than about 0.5 nm, less than about 0.2 nm, or having a thicknessthat corresponds to essentially a single monolayer sufficient topassivate essentially all dangling bonds proximate the semiconductorsurface.

Further, thin interface layers may be advantageously grown on asemiconductor in the presence of nitrogen gas, or other inertnitrogen-containing gas. The reaction rate of a semiconductor such assilicon with nitrogen gas is significantly lower than that of a reactivenitrogen-containing gas such as ammonia. The slow growth rate may bedesirable for better control of the growth of films of nitrogen on asemiconductor of a thickness of less than about 1 nm, less than about0.5 nm, less than about 0.2 nm, or having a thickness that correspondsto essentially a single monolayer sufficient to passivate essentiallyall dangling bonds proximate the silicon surface.

IV. Diodes Containing Passivated Semiconductor Surfaces

Diodes made from Schottky barriers (i.e., asymmetric electricalpotentials formed at a junction between a metal and a semiconductor) arewidely used in rectifiers in power supply and control applications. Asused herein, the terms Schottky diode, metal-semiconductor junctiondiode, diode, and rectifier are all related and appear in order frommore specific at the left to more general at the right. Likewise, theterms Schottky barrier, metal-semiconductor barrier,conductor-semiconductor junction, and multi-material junction are allrelated and appear in order from more specific at the left to moregeneral at the right. The term Schottky diode will be used to refer to adiode containing a Schottky barrier.

As mentioned above, the present inventors have devised a scheme tocontrol or adjust a Schottky barrier height by forming an interfacelayer (which includes or sometimes consists of a passivation layer thatincludes an oxide, oxynitride, nitride, arsenide, hydride, fluoride, oran equivalent) between a metal and a semiconductor. This scheme differsfrom past attempts by others to control barrier height, which attemptsgenerally involved either using a silicide as a contact metal (and thuslimiting the choices of available contact metals to those that can formsilicides), or using esoteric substrates that exhibit wide bandgaps.Further, in previous devices the Fermi level of the semiconductorremains pinned, with the barrier height being virtually independent ofthe metal used. Finally, doping of substrates has also been attempted,however, it has not been shown to truly affect the barrier height of thesubstrate material. For example, PtSi contacts have reduced resistancedue to high silicon doping such that the current across the junction isdominated by tunneling through the barrier. Doping may thus lead tocases where the top of the barrier may be so thin as to be essentiallytransparent to electrons, however, doping does not appear to allowactual tuning of the barrier height.

FIG. 6 shows an example of a diode 600 containing, according to oneembodiment of the present invention, an interface layer 620 disposedbetween and attached to both a semiconductor 610 and a conductor 630.The conductor and the semiconductor are operable to be electricallycoupled with different voltages associated with the operation of thediode 600 and to pass electrical current through a passivatedsemiconductor surface formed at the junction between the semiconductor610 and the interface layer 620.

The conductor 630 contains a conductive material such as a metal or analloy of a metal. The terms metal, conductive material, and conductorare all related and appear in order from specific at the left to generalat the right. In general, the terms refer to a highly electricallyconductive substance that has a Fermi energy level that sits in apartially filled band. Unless otherwise specified, conductors includemetals (e.g., pure metals and alloys), and other conductors such asdoped polysilicon (a nonporous silicon containing randomly orientedcrystallites), doped single crystal silicon, and metal silicides. Notethat alloys may have workfunctions different than their constituents andmay be designed to have specific workfunctions though selective use ofratios of the constituent metals.

Often, the conductor is a metal since metals may offer advantages overconductive semiconductors including lower resistance, higher carriermobilities that provide superior high frequency performance andswitching, favorable low power characteristics, and ease ofmanufacturing control. Use of metals may also avoid the need to performsemiconductor doping, which may simplify manufacturing and improvequality control.

Metals that are contemplated include pure metals, alloys, refractorymetals, metals that do not form silicides, metals physically depositedby substantially non-invasive processes such as by condensation of athermally evaporated metal vapor, and metals having a predetermined workfunction. The use of non-invasively deposited metals may allow forforming the metal on a thin interface layer without disrupting thepassivation properties of the layer.

A metal having a predetermined work function may be a metal having awork function smaller or greater than that of the semiconductor,depending on the desired application. Often, the semiconductor will besilicon. In this case by the work function of a semiconductor or siliconwe mean the energy in the middle of the semiconductor bandgap. Exemplarymetals that have a work function smaller than silicon include Group 3Aelements, aluminum (Al), indium (In), titanium (Ti), chromium (Cr),tantalum (Ta), cesium (Cs), magnesium (Mg), erbium (Er), ytterbium (Yb),manganese (Mn), lead (Pb), silver (Ag), yttrium (Y), and zinc (Zn).Exemplary metals that have a work function greater than silicon includeplatinum (Pt), gold (Au), tungsten (W), nickel (Ni), molybdenum (Mo),copper (Cu), cobalt (Co), and palladium (Pd).

The semiconductor-interface layer-conductor configuration illustrated inFIG. 6 defines what the present inventors have chosen to call a“passivated Schottky barrier”. The passivated Schottky barrier is anaturally formed electrical potential barrier to an electron or hole atthe Fermi energy (the electrochemical potential) in the conductor due toa depletion region formed in the semiconductor adjacent the conductor.The passivated Schottky barrier may deviate in barrier height from astandard un-passivated Schottky barrier that would form naturally at acontact junction between the semiconductor and the conductor without theinterface layer disposed therebetween. That is, the passivated Schottkybarrier may have a barrier height that depends predominantly upon thebulk characteristics of the semiconductor and the conductor, rather thanon surface properties, and may depend in part on the characteristics ofthe interface layer.

Indeed, the present inventors have determined that changes in barrierheight are approximately monotonic and continuous for variations insurface passivation thickness by nitridation of the semiconductorsubstrate. More specifically, experiments by the present inventors in aregime where the nitride layer is sufficiently thick to remove show thattemperature of interface layer formation has the strongest effect onbarrier height. In other regimes, thickness may be critical. Ideally, ifall surface states are removed, barrier height should be controllablesimply by the choice of metal used.

To understand why thickness of the interface layer is important, referbriefly to FIG. 8 where a graph of interface-specific contact resistanceversus interface thickness is shown. The graph is for a structure wherethe workfunction of the metal is the same as the electron affinity inthe semiconductor, such that the Fermi level of the metal lines up withthe conduction band of the semiconductor. At large thicknesses, theinterface layer poses significant resistance to current. As thicknessdecreases, resistance falls due to increased tunneling current. However,there comes a point where even as the interface layer continues to getthinner, resistance increases. This is due to the effect of MIGS, whichincreasingly pull the Fermi level of the metal down towards mid-gap ofthe semiconductor, creating a Schottky barrier. The present inventorshave discovered that this competition results in an optimum thickness,as shown in the illustration, where the resistance is a minimum. At thisthickness the effect of MIGS has been sufficiently reduced to tiepin themetal and lower the Schottky barrier, and the layer is stillsufficiently thin to allow significant current flow across the interfacelayer. Contact resistances of less than or equal to approximately 2500Ω-μm², 1000 Ω-μm², 100 Ω-μm², 50 Ω-μm², 10 Ω-μm² or even less than orequal to 1 Ω-μm² may be achieved.

Characteristics that may be adjusted to provide a desired barrier heightthus include the passivation material used (e.g., selection based onbandgap), the interface layer thickness (e.g., especially where theinterface layer is a compound layer formed of a passivation layer and aseparation layer), the method of forming the interface layer (e.g.,control of parameters such as temperature), the interface layerthickness that is substantially similar to a penetration depth of MIGSformed at a metal interface, the metal used for the source and/or drain,and other characteristics.

One advantage of the ability to adjust the Schottky barrier height withthe introduction of interface layer 620 is the ability to form asubstantially high barrier height. For example, an interface layer maybe used to create a Schottky barrier having a barrier height that isgreater than can be achieved through the use of metal silicides, greaterthan about 2.0 eV, or greater than about 2.5 eV (using a semiconductorwith a bandgap at least this large), or nearly 1.0 V using silicon. Suchhigh barrier heights imply the ability to withstand high voltages beforebreakdown occurs. Thus, Schottky barriers having such high barrierheights may be particularly useful in high-voltage Schottky diodes.

Another advantage achieved through the use of the interface layer 620 isgreater flexibility afforded in selecting a conductor 630. Typically,metals chosen for application in classic Schottky diodes are those thatcan form a silicide with a silicon semiconductor. The formation of thesilicide helps to reduce surface states (resulting from dangling bonds),but not the effects of MIGS. Thus, the Fermi level at the semiconductorsurface is still pinned. Using metals that form silicides upon contactwith silicon may thus help to make the devices more reproducible in amanufacturing environment, but such devices still suffer from thedrawback of having a barrier height that is fixed.

According to one embodiment of the present invention, however, one mayselect a conductor that is not able (or not readily able) to form asilicide with the semiconductor. The metal silicide is not neededbecause the interface layer provided in accordance with the presentinvention passivates the semiconductor surface and also reduces oreliminates the effect of MIGS. This may allow for selection of a metalthat has properties such as a desirable work function or Fermi levelenergy, even though that metal may not form a metal silicide.

For example, to make large-barrier diodes, for an n-type doped siliconsemiconductor, a metal may be selected that has a work function that iseither substantially equal to the valence band energy of thesemiconductor or that is within about 0.1 eV to about 0.3 eV of thevalence band energy of the semiconductor. Similarly, for a p-type dopedsilicon semiconductor, a metal may be selected that has a work functionsubstantially equal to the conduction band energy of the semiconductor.For Schottky diodes configured in accordance with the present invention,the Fermi level of the metal may lie anywhere in the bandgap of thesemiconductor when an interface layer is disposed within the junction,resulting in diodes of various barrier heights. The Fermi level of themetal may also lie in the conduction or valence band of thesemiconductor.

The use of interface layer 620 thus provides a way to tune, adjust, orcontrol the height of the barrier between the conductor and thesemiconductor. Without the interface layer 620, the barrier height wouldbe substantially un-tunable, un-adjustable, and fixed (as discussedabove).

The role played by interface layer 620 in tuning, adjusting, orcontrolling the height of the barrier between the conductor 630 and thesemiconductor 610 may be understood as a depinning of the Fermi level ofthe semiconductor. That is, the interface layer may reduce surfacestates by bonding to the semiconductor material to consume danglingbonds. Additionally, the interface layer may reduce the formation ofMIGS in the semiconductor by providing a thickness and bandgap thatprevent the electron wave function (of the metal) from penetrating intothe semiconductor. The electron wave function may instead penetrate intothe interface layer and form MIGS within the interface layer at anenergy related to the states of the interface layer material. Asdesired, the density of the MIGS and the depth of MIGS penetration intothe interface layer may be reduced by choosing an interface layermaterial or materials having a larger bandgap or higher effective massthan the semiconductor.

According to one embodiment of the present invention then, the interfacelayer 620 is incorporated into a device operable to pass current throughthe semiconductor surface and the interface layer during deviceoperation. In such an embodiment, it may be desirable to use aninterface layer having a thickness of a monolayer, or, for examplebetween about 0.1 nm and about 0.3 nm, and also having a wide bandgap(as compared to that of the semiconductor) so that the interface layerboth de-pins the Fermi level (so that the barrier height dependspredominantly on bulk properties of the junction materials) and allowssufficient current transfer across it. Advantageously, such interfacelayers may be sufficiently thin to provide low impedance to current flow(due to the exponential dependence of direct tunneling on barrierthickness), which is desirable for many semiconductor devices, whilealso providing sufficient semiconductor surface passivation to allow anadjustable barrier height. That is, the interface layer may allowpassivation of surface states and reduction (or elimination) of MIGS inthe semiconductor to allow for an adjustable barrier height with asubstantially thin layer that allows sufficient current to betransferred across the interface layer.

There are several methods by which the barrier height can be madeadjustable. For example, adjustment may be made by tuning the degree ofFermi level pinning. In other words, some embodiments may allow for asufficiently thin interface layer so that not all of the effects of MIGSin the Si are eliminated. Further, the pinning may be varied bycombinations of thickness of the interface layer and the choice ofinterface material. The metal in contact with the interface layer may bepinned by MIGS at different levels in different materials. Conversely,or in addition, the passivation may be left incomplete to allow for aneffective level of unpassivated states. Complete depinning of the Fermilevel (that is removal of all surface states in Si including MIGS) isanother option, in which case one could tune the barrier height simplyby choosing a pure metal or an alloy that possesses the desiredworkfunction. In that case, the barrier height is determined by Equation(1), which until now has been an unrealizable idealization. Note thatthe type of tuning being discussed here is adjustment of the barrierheight by altering the structure of the junction at the time ofmanufacture, not by varying an externally applied condition duringjunction operation.

FIG. 7a-7d show relationships between Fermi energy, conduction bandenergy, and valence band energy for various Schottky barriers containinga metal in contact with (or in close proximity to) a semiconductor,where the bandgap (E_(g)) of the semiconductor exists between theconduction band (E_(c)) and the valence band (E_(v)). In this example,the work function of the metal Φ_(M) is chosen to be approximately equalto the electron affinity χ_(S) of the semiconductor. In FIG. 7a , anunpassivated Schottky barrier 700 is shown. In this example, the Fermilevel (E_(F)) of the metal 730 is pinned in the bandgap of thesemiconductor 710. This results in a discontinuity in the vacuum levelcaused by a charged dipole at the interface.

In FIG. 7b , the interface layer 720 b is thick enough to passivatedangling bonds at the surface of the semiconductor 710, but not thickenough to eliminate or sufficiently reduce the effect of MIGS. As aresult, the band structure is largely unaltered from that seen in theprevious illustration. Similarly, in FIG. 7c , when the interface layer720 c is sufficiently thick to eliminate or reduce the effect of MIGSbut not to passivate the semiconductor surface, little change in theenergy band structure is observed. However, as shown in FIG. 7d , whenthe interface layer 720 d is sufficient to both eliminate or reduce theeffect of MIGS and to passivate the semiconductor surface, we see theFermi level of the metal aligning with the conduction band of thesemiconductor (i.e., the Fermi level of the semiconductor has beendepinned and no longer lines up with the Fermi level of the metal). Thevacuum level is now continuous as there is no charged dipole at theinterface. Thus, the band structure of a device constructed in thisfashion is a result of only bulk material properties, not properties ofthe surface. By way of example, the materials in such cases may be Aland Si, with a work function for Al of approximately Φ_(M)=4.1 eV andthe electron affinity for Si of approximately χ_(S)=4.05 eV.

V. Transistors Containing Passivated Semiconductor Surfaces

The interface layers described herein may be used in connection with asemiconductor surface of a channel in a field effect transistor. Thatis, an interface layer may be disposed between a source and a channel, achannel and a drain, or both of an insulated gate field effecttransistor. Such use of an interface layer is described in detail inco-pending U.S. Pat. No. 6,833,556, issued Dec. 21, 2004, entitled“INSULATED GATE FIELD EFFECT TRANSISTOR HAVING PASSIVATED SCHOTTKYBARRIERS TO THE CHANNEL”, filed Jan. 14, 2003 by the present inventors,and assigned to the assignee of the present invention.

The source and drain contacts at the channel of a field effecttransistor are examples of a broader category of metal-interfacelayer-semiconductor contacts that make up the present invention. In thepast, such contacts generally comprised a silicide-n+-Si junction, whichformed a somewhat “leaky” Schottky diode, with a Fermi level of thesemiconductor pinned at the midgap. In contrast, the present inventionprovides a contact wherein the Fermi level of the metal is aligned withthe conduction band of the semiconductor (e.g., as shown in FIG. 7d ).Note that in other cases, depending on the type of semiconductormaterial and conductors used, the Fermi level of the metal may alignwith the valence band of the semiconductor.

Although both types of junctions (i.e., the new passivated Schottkybarrier junction and the conventional silicide-semiconductor junction)permit tunneling currents, the present junction can be fabricated with amuch thinner interface layer as compared to the thickness of thesilicide layer used previously. Indeed, thickness of an order ofmagnitude less than the silicide thickness can be expected. In aconventional silicide semiconductor junction a Schottky barrier isformed which is comprised of a depletion layer. The tunnel barrierpresented by such a depletion layer may be an order of magnitude thickerthan the dielectric tunnel barrier in the present invention. The thinnerinterface layers provided by the present invention permit higher currentacross the junction (i.e., lower junction specific contact resistance).

Two other properties of the dielectric deserve mention. First is theproperty of the height of the barrier compared to the semiconductorconduction band (for electrons). In making the barrier thinner than asilicide barrier, the tradeoff may be a higher tunnel barrier (e.g., 2eV for nitride, compared with about half the gap of 0.6 eV forsilicide). Spacer layers may be used with lower barriers (e.g., TiO₂ hasa barrier of less than 1 eV). Nevertheless, even with the higher barrierto electrons, the present inventors have determined that the resistancecan still be one hundred times lower than a contact to silicon with asilicide barrier.

The second property is the effective mass of electrons in thedielectric. Larger mass electrons will not penetrate as far (i.e.,because of their shorter wavelength) from the metal into thesemiconductor. The less the electrons penetrate into the dielectric, theless the effect of MIGS in the dielectric. Thus, MIGS in the dielectricare reduced with larger bandgap and larger effective mass.

In addition the junction of the present invention can be used in makingcontacts to source or drain implanted wells and will have the advantageof reducing the need for high doping levels (which are now reachingtheir limits of solid solubility). The high doping profiles wererequired in the past in order to keep the junction depletion layerrelatively thin, so as to increase the tunneling current, thus reducingthe junction resistance. However, it is becoming increasingly difficultto increase doping profiles in order to provide low resistancejunctions. It may be possible to reach the same level of resistance witha lower doping concentration using the present invention. It may furtherbe possible to achieve much lower resistance even with lower dopingconcentration. When the present invention is used with high dopinglevels, the resistance will be further reduced.

Thus, methods and applications for semiconductor-interface layer-metaljunctions have been described. Although described with reference tospecific embodiments it should be remembered that various modificationsand changes may be made to the techniques described herein withoutdeparting from the broader spirit and scope of the invention. Thespecification and drawings are accordingly to be regarded in anillustrative rather than a restrictive sense and the invention measuredonly in terms of the claims, which follow.

What is claimed is:
 1. An electrical junction, comprising an interfacelayer disposed between and in contact with a conductor and an n-typedoped semiconductor, the interface layer comprising an element having avalence equivalent to that of arsenic and configured to reduce a heightof a Schottky barrier between the conductor and the semiconductor fromthat which would exist at a junction between the conductor and thesemiconductor without the interface layer disposed therebetween.
 2. Theelectrical junction of claim 1, wherein the conductor comprises a metalsilicide.
 3. The electrical junction of claim 1, wherein the interfacelayer comprises a monolayer of an element having a valence equivalent tothat of arsenic.
 4. The electrical junction of claim 3, wherein theconductor comprises a metal silicide.
 5. The electrical junction ofclaim 1, wherein the n-type doped semiconductor comprises n-type dopedsilicon.
 6. The electrical junction of claim 1, wherein the n-type dopedsemiconductor comprises n-type doped germanium.
 7. The electricaljunction of claim 1, wherein the n-type doped semiconductor comprises asource of a transistor.
 8. The electrical junction of claim 7, whereinthe n-type doped semiconductor comprises n-type doped silicon.
 9. Theelectrical junction of claim 1, wherein the n-type doped semiconductorcomprises a drain of a transistor.
 10. The electrical junction of claim1, wherein the electrical junction has a specific contact resistance ofless than or equal to approximately 100 Ω-μm2.
 11. The electricaljunction of claim 1, wherein the electrical junction has a specificcontact resistance of less than or equal to approximately 10 Ω-μm2. 12.The electrical junction of claim 1, wherein the electrical junction hasa specific contact resistance of less than or equal to approximately 1Ω-μm2.
 13. A field effect transistor, comprising a source conductor, adrain conductor, a semiconductor channel region between the sourceconductor and the drain conductor, a gate proximate the semiconductorchannel region, and an interface layer comprising an element having avalence equivalent to that of arsenic between the source conductor andthe semiconductor channel region.
 14. The field effect transistor ofclaim 13, wherein the source conductor comprises a metal silicide. 15.The field effect transistor of claim 13, wherein the interface layercomprises a monolayer of an element having a valence equivalent to thatof arsenic.
 16. The field effect transistor of claim 15, wherein thesource conductor comprises a metal silicide.
 17. The field effecttransistor of claim 13, wherein the semiconductor channel regioncomprises silicon.
 18. The field effect transistor of claim 13, whereinthe semiconductor channel region comprises silicon germanium.
 19. Thefield effect transistor of claim 13, wherein the semiconductor channelregion comprises germanium.
 20. An electrical junction, comprising aninterface layer disposed between and in contact with a conductor and ann-type semiconductor, the interface layer comprising an element having avalence equivalent to that of arsenic and an oxide of titanium.